1. Field of the Invention
The invention relates to a hydraulic actuator for a gas exchange valve for internal combustion engines.
2. Description of the Prior Art
The opening and closing of the gas exchange valve should be as fast as possible, in order to minimize flow losses from the gas exchange valve either when the combustion air is aspirated or upon expulsion of the exhaust gases from the combustion chamber.
The overpressure intermittently prevailing in the combustion chamber of the engine presses the gas exchange valve into the valve seat. Because of this overpressure, opening the gas exchange valves requires an increased expenditure of force for lifting the gas exchange valve, in particular the outlet valve, from the valve seat. Once the gas exchange valve has lifted from the valve seat, the pressure in the combustion chamber drops sharply, so that the force needed to open the gas exchange valve is correspondingly less.
Upon closure of the gas exchange valve, it must also be noted that the speed at which the valve plate of the gas exchange valve strikes the valve seat should not be excessive. If that speed is too high, unwanted noise and increased wear occur when the valve plate strikes the valve seat.
The object of the invention is to furnish a hydraulic actuator for a gas exchange valve which can exert a strong force at the onset of the opening motion on the gas exchange valve, which enables fast control motions of the gas exchange valve, and in which the gas exchange valve strikes the valve seat at low speed.
According to the invention, this object is attained by a hydraulic actuator for a gas exchange valve of an internal combustion engine, having a cylinder bore, having a piston, and having an annular piston, the piston and the annular piston being guided in the cylinder bore, and the piston, annular piston and cylinder bore define a first chamber in the axial direction whose volume increases when the actuator opens the gas exchange valve, and the annular piston and the cylinder bore define a second chamber in the axial direction whose volume decreases when the actuator opens the gas exchange valve, and the piston and the cylinder bore define a third chamber whose volume decreases when the actuator opens the gas exchange valve, and having a device for limiting the volumetric decrease of the second chamber.
In the hydraulic actuator of the invention, at the onset of the opening motion of the gas exchange valve, a strong hydraulic force is transmitted by the actuator to the gas exchange valve, so that despite the contrary pressure on the valve plate of the gas exchange valve from the combustion chamber, the gas exchange valve can be lifted securely and quickly from the valve seat. As soon as the force needed to actuate the gas exchange valve has decreased, for instance because there is no longer any substantial contrary pressure in the combustion chamber, the annular piston is no longer moved onward, and consequently only a lesser hydraulic force is now exerted on the piston of the actuator, and this lesser force is transmitted in turn to the gas exchange valve. With the reduction in the hydraulic force, the energy required to adjust the actuator piston is also reduced, so that the overall energy required for valve control of the engine drops. Simultaneously with the reduction in this force, the adjusting speed of the gas exchange valve also varies. Finally, upon closure of the gas exchange valve, braking of the gas exchange valve by the hydraulic actuator of the invention can be achieved before the gas exchange valve strikes the valve seat of the engine. This reduces the wear to the valve seat and gas exchange valve and also lessens the noise produced by the valve control of the engine.
The onset of the braking operation of the gas exchange valve upon its closure is moreover independent of production tolerances in the gas exchange valve and of the temperature-caused changes in length that always exist in internal combustion engines because of thermal expansion. With the actuator of the invention, highly stable operation of the engine can therefore be achieved and is affected by neither temperature expansions nor production tolerances.
In a variant of the invention, it is provided that the piston has a plunge cut; that the annular piston has a stepped center bore with one larger diameter and one smaller diameter; and that the annular piston can be slipped by the larger diameter of the center bore onto the piston, so that the ratio of the actuating forces of the actuator upon opening of the gas exchange valve and during the remaining adjusting motion is adjustable in a simple way.
This effect can be further enhanced by providing that the diameters of the piston on both sides of the plunge cut are different; and that the annular piston can be slipped onto the larger diameter.
In a further feature of the invention it is provided that the device for limiting the volumetric reduction of the second chamber is a pressure reservoir that is in communication with the second chamber and that has a piston; and that the travel of the piston is limitable, so that the annular piston can be arrested in a simple way by hydraulic means. Since the pressure reservoir does reach the high temperatures of the gas exchange valve and the cylinder head of the engine, the position in which the annular piston is arrested after the gas exchange valve has opened is independent of the thermal expansions of the gas exchange valve and of the cylinder head.
Further features of the invention provide that the pressure reservoir is a spring reservoir or a gas reservoir, and/or that the travel of the piston is limitable by a stop, in particular an adjustable stop, so that the actuator of the invention can be adjusted simply.
Further features of the invention provide that the first chamber can be made to communicate with a pump via a first switching valve; that the second chamber can be made to communicate with an oil pump via a second switching valve; and that the third chamber is acted upon by the feed pressure of the pump, so that by the actuation of two switching valves, the gas exchange valve can either be opened or closed by the hydraulic actuator of the invention, and the increased force upon liftoff of the gas exchange valve from the valve seat and the slowing down of the gas exchange valve before it strikes the valve seat can be realized automatically by the hydraulic actuator of the invention.
Separate triggering of the hydraulic actuator for that purpose is unnecessary. This makes the work of the control unit required for triggering the actuator easier, and makes the hydraulic actuator of the invention robust and insensitive to external factors.
The action according to the invention of the actuator is further reinforced by the provision that the first chamber and the second chamber are hydraulically in communication with one another via a throttle, in particular an adjustable throttle, and/or that a check valve is provided between the second chamber and the first chamber and blocks the hydraulic communication from the first chamber to the second chamber. The throttle has a definitive influence on the braking of the gas exchange valve before it strikes the valve seat.
In an advantageous embodiment of the invention, the device for limiting the volumetric decrease in the second chamber has a shutoff valve which is in communication with an opening in the second chamber and which in one switching position closes the opening and in its other switching position opens it to allow fluid to flow out. With the closure of the shutoff valve, the annular piston is fixed, so that the instant of closure of the shutoff valve defines the stroke length of the annular piston. The instant of onset of the braking action upon closure of the gas exchange valve is in turn dependent on the stroke length of the annular piston; this braking action ensues earlier with a longer stroke of the annular piston and later with a shorter stroke. Thus by means of the shutoff valve, the onset of braking can be adjusted independently of production tolerances or material expansions caused by temperature fluctuations.
In an advantageous embodiment of the invention, the shutoff valve is not used as an additional component unit; instead, its function is allocated to the second switching valve, which is required anyway to initiate the closing operation of the gas exchange valve. With the omission of the shutoff valve and by dispensing with the above-described pressure reservoir for the device for limiting the volumetric decrease in the second chamber, the construction costs for valve control are reduced.
In an advantageous embodiment of the invention, between the first chamber and the throttle disposed between the two chambers for varying the braking behavior of the actuator piston and thus of the gas exchange valve, a flow-controlled valve is provided which is embodied such that it is closable by the fluid flowing to the first chamber. This has the advantage that in the initial phase of the stroke of the actuator piston, in which both switching valves are open, fluid from the first switching valve cannot flow directly via the throttle into the hydraulic relief chamber or oil sump. It is true that if the throttling action of the throttle is strong, this flow-controlled valve can be dispensed with, since only slight quantities of fluid flow out via the throttle; however, if there is a relatively large throttle opening for the sake of attaining an only slight braking action at the gas exchange valve, then the flow-controlled valve is indispensable for blocking off the throttle, if major leakage is to be avoided.